From d69804b540d0bf168f0706af289ea3583353a30c Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter@eisentraut.org>
Date: Wed, 14 Aug 2024 08:50:15 +0200
Subject: [PATCH] Add prefetching support on macOS

macOS doesn't have posix_fadvise(), but fcntl() with the F_RDADVISE
command does the same thing.

TODO: docs update
---
 src/backend/commands/variable.c |  4 +--
 src/backend/storage/file/fd.c   | 52 +++++++++++++++++++++++----------
 src/include/pg_config_manual.h  |  7 ++---
 3 files changed, 41 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/variable.c b/src/backend/commands/variable.c
index 6202c5ebe44..c1c6c2811c9 100644
--- a/src/backend/commands/variable.c
+++ b/src/backend/commands/variable.c
@@ -1212,7 +1212,7 @@ check_effective_io_concurrency(int *newval, void **extra, GucSource source)
 #ifndef USE_PREFETCH
 	if (*newval != 0)
 	{
-		GUC_check_errdetail("\"effective_io_concurrency\" must be set to 0 on platforms that lack posix_fadvise().");
+		GUC_check_errdetail("\"effective_io_concurrency\" must be set to 0 on platforms that lack prefetching support.");
 		return false;
 	}
 #endif							/* USE_PREFETCH */
@@ -1225,7 +1225,7 @@ check_maintenance_io_concurrency(int *newval, void **extra, GucSource source)
 #ifndef USE_PREFETCH
 	if (*newval != 0)
 	{
-		GUC_check_errdetail("\"maintenance_io_concurrency\" must be set to 0 on platforms that lack posix_fadvise().");
+		GUC_check_errdetail("\"maintenance_io_concurrency\" must be set to 0 on platforms that lack prefetching support.");
 		return false;
 	}
 #endif							/* USE_PREFETCH */
diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index 3944321ff37..8fe3ac58d97 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -2068,7 +2068,9 @@ FileClose(File file)
 /*
  * FilePrefetch - initiate asynchronous read of a given range of the file.
  *
- * Currently the only implementation of this function is using posix_fadvise
+ * Returns 0 on success, otherwise an errno error code.
+ *
+ * XXX Currently the only implementation of this function is using posix_fadvise
  * which is the simplest standardized interface that accomplishes this.
  * We could add an implementation using libaio in the future; but note that
  * this API is inappropriate for libaio, which wants to have a buffer provided
@@ -2077,31 +2079,51 @@ FileClose(File file)
 int
 FilePrefetch(File file, off_t offset, off_t amount, uint32 wait_event_info)
 {
-#if defined(USE_POSIX_FADVISE) && defined(POSIX_FADV_WILLNEED)
-	int			returnCode;
-
 	Assert(FileIsValid(file));
 
 	DO_DB(elog(LOG, "FilePrefetch: %d (%s) " INT64_FORMAT " " INT64_FORMAT,
 			   file, VfdCache[file].fileName,
 			   (int64) offset, (int64) amount));
 
-	returnCode = FileAccess(file);
-	if (returnCode < 0)
-		return returnCode;
+#if defined(USE_POSIX_FADVISE) && defined(POSIX_FADV_WILLNEED)
+	{
+		int			returnCode;
+
+		returnCode = FileAccess(file);
+		if (returnCode < 0)
+			return returnCode;
 
 retry:
-	pgstat_report_wait_start(wait_event_info);
-	returnCode = posix_fadvise(VfdCache[file].fd, offset, amount,
-							   POSIX_FADV_WILLNEED);
-	pgstat_report_wait_end();
+		pgstat_report_wait_start(wait_event_info);
+		returnCode = posix_fadvise(VfdCache[file].fd, offset, amount,
+								   POSIX_FADV_WILLNEED);
+		pgstat_report_wait_end();
 
-	if (returnCode == EINTR)
-		goto retry;
+		if (returnCode == EINTR)
+			goto retry;
 
-	return returnCode;
+		return returnCode;
+	}
+#elif defined(__darwin__)
+	{
+		struct radvisory
+		{
+			off_t		ra_offset;	/* offset into the file */
+			int			ra_count;	/* size of the read     */
+		}			ra;
+		int			returnCode;
+
+		ra.ra_offset = offset;
+		ra.ra_count = amount;
+		pgstat_report_wait_start(wait_event_info);
+		returnCode = fcntl(VfdCache[file].fd, F_RDADVISE, &ra);
+		pgstat_report_wait_end();
+		if (returnCode != -1)
+			return 0;
+		else
+			return errno;
+	}
 #else
-	Assert(FileIsValid(file));
 	return 0;
 #endif
 }
diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h
index b769030d8fa..84bb5368b3d 100644
--- a/src/include/pg_config_manual.h
+++ b/src/include/pg_config_manual.h
@@ -138,12 +138,9 @@
 
 /*
  * USE_PREFETCH code should be compiled only if we have a way to implement
- * prefetching.  (This is decoupled from USE_POSIX_FADVISE because there
- * might in future be support for alternative low-level prefetch APIs.
- * If you change this, you probably need to adjust the error message in
- * check_effective_io_concurrency.)
+ * prefetching.
  */
-#ifdef USE_POSIX_FADVISE
+#if defined(USE_POSIX_FADVISE) || defined(__darwin__)
 #define USE_PREFETCH
 #endif
 
-- 
2.46.0